From StarWars.com:
"The Keeper of the Jedi Archives has entered a new holorecord. Continuing his investigation, Master Gnost-Dural delves deeper into the history of the galaxy after discovering that the blockade that Hylo Visz helped end was a result of the actions of the new Mandalore.
During a Republic resurgence in the Great War, a bold young warrior rose to fame in the Gladiatorial arenas on Geonosis. Adopting the ancient and infamous title of Mandalore, this young leader amassed an army of Mandalorian mercenaries and bounty hunters. The new Mandalore led his forces to form a blockade and challenge the Jedi Order, effectively turning the tide of the war back in the Sith Empire's favor.

Watch the third Timeline holorecord, The Return of the Mandalorians, narrated by actor Lance Henriksen at the official site for Star Wars: The Old Republic[here]."